FLIGHT International, II April 1981 1039 I
ItBffiViiT. FS
Nasa tests Columbia's engines during a
20sec flight readiness firing, which took place
during February
WHEN Space Shuttle's Solid Rocket Boosters and main
engines are finally lit, almost every
thing has to work correctly. The lives
of Commander John Young and pilot
Robert Crippen, as well as the future
of American spaceflight, depend on it.
Space Shuttle is the first recusable
rocket and none of it has flown un
manned tests in space;—as happened
with Mercury, Gemini and Apollo. It
is the first time that solid-propellant
boosters have been used on a manned
spacecraft, the first bluff-Shape rocket,
the pioneer of throttleable engines,
and will be the first US manned flight
for almost six years.
The countdown for the first flight
begins roughly 73hr before lift-off
(T-73hr). The launch window is dic
tated by the need for good light
during ascent and landing in normal
flight, or during an abort. It opens at
sunrise plus 45min and is more than
6hr long. Nasa is currently aiming for
launch on April 10, for which the
window opens at 06.48hr Eastern day
light time.
Young and Crippen will enter
Columbia's cockpit about two hours
before lift-off. They will be wired-up
with sensors to measure heart-beat
and respiration, and will be wearing
g-suits and ejection suits. Columbia's
ejection seats are intended for use in
the four Shuttle test flights at speeds
of up to Mach 2-7 and a height of
30,480m, and the suits are worn dur
ing launch and entry. An independent
air supply toi the suit provides protec
tion against loss of cabin pressure.
During lift-off the astronauts are
in a supine sitting position. Their
main task during launch will be to
monitor the computer-controlled flight
of the Shuttle—intervening only
when something goes wrong.
Space Shuttle Main Engine (SSME)
ignition begins 3-5sec before lift-off,
but a shutdown will occur if any
engine fails to develop at least 90
per cent power—each should be pro
ducing 100 per cent. If the SSMEs
are go, the SRBs are lit and their
hold<lown clamps released. Roughly
0 • 3sec later the Space Shuttle thrust/
weight ratio becomes greater than
one and lift-off occurs. It takes 6sec
for Columbia's SSMEs to clear the
anti-lightning tower atop pad 39A,
during which the Shuttle rises verti
cally (a list of important events and
tames appears on page 1040).
As soon as the tower is clear a
gradual pitch-over begins to point the
astronaut's heads toward Earth, so
that by the time they reach orbit
they are hanging upside-down in their
seats. (Pitch-over is necessary with
any orbital attempt, to build up hori
zontal speed by the time injection
height is reached). Nasa has chosen
to make the Shuttle pitch over on to
its back rather than its front for
_.. _._ FT&SSB
by DAVID
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two main reasons—loads between the
Orbiter and ET are lower, and the
crew can see the Earth better.
The next major event is a reduc
tion in SSME thrust from 100 to
65 per cent to minimise the maximum
dynamic pressure—the aerodynamic
load caused as the craft builds up
speed in the comparatively thick
lower atmosphere. SRB separation
happens automatically just over two
minutes after Mft-off. By this time
Columbia's three SlSMEs are capable
of powering it and the ET on their
own—much of the liquid oxygen and
hydrogen originally in the ET having
been consumed.
The spent SRBs follow a parabolic
trajectory toward the Atlantic Ocean.
Parachutes are deployed when the
SRBs have fallen to a height of about
4,700m, slowing their fall to a gentle
splash in the sea 255km downrange.
The SRBs carry beacons which help
Nasa's two recovery ships to find
them, and the ships then retrieve the
parachutes and tow the empty SRBs
back to the Kennedy Space Centre
(KSC), Florida. When the SRBs arrive
they will be washed and sent back
to the manufacturer, who will refur
bish them for another flight.
At almost eight minutes into the
flight SSME thrust is again reduced.
This time the reduction is to limit
accelerations to 3g, which is one of
the Shuttle-design criteria. Shortly
afterward thrust is reduced to its
minimum value of 65 per cent and the
SiSMEs are shut down.
ET separation occurs a few seconds
later—'provided that valves in the
liquid oxygen and hydrogen pipes be
tween Columbia and the ET are shut,
and that pitch, roll and yaw-rate are
less than 0-5°/sec. These conditions
are necessary to avoid the ET hitting
Columbia when they part. Columbia
is nudged clear of the ET by its re
action control system (RCS) a frac
tion of a second after separation. A
further evasive manoeuvre is then